Many hydraulic devices such as proportional control valves require pilot stage or hydraulic amplifiers between the input or electric signal and the hydraulic fluid being controlled. The electric signal cannot directly generate sufficient force to move the valve components of the hydraulic device rapidly in response to the changes of signal input. Generally the electric input signal is directed to a solenoid or torque motor which is used to shift a pilot spool, flap or valve jet nozzle or similar device, a number of which are described in Fluid Power Handbook and Directory, 1990-91 A/94-A/107. The hydraulic fluid controlled in this manner is used to shift the spool of the main valve or some other component in the main valve which controls the main fluid flow to the device being controlled, i.e. the main valve controls the flow of a significantly higher amount of fluid at a significantly higher pressure than that used for example in a hydraulic pilot valve.
Typically the pressure in the hydraulic pilot valve are an order of magnitude smaller than the pressure in the main hydraulic circuit. Further, the passage sizes within the hydraulic pilot are small rendering the pilots sensitive to contaminants within the fluid and requiring, in some cases, special filtering of the hydraulic fluid. It is also important that the tolerances to which the hydraulic pilot is machined be relatively precise thereby increasing the cost.
It is has also been proposed to use a controlled pulse-width-modulation system, i.e. one in which a pilot signal controls a pair of fast on/off solenoid valves which apply depending on which one of the valves is actuated, a supply pressure and then a tank pressure to the hydraulic pilot circuit. The fraction of the time in each of these cycles when the circuit is connect to the supply versus the time it is connected to the tank pressure, determines the pressure within the pilot circuit (see for example U.S. Pat. No. 4,813,339 issued Mar. 21, 1989 to Hitachi Limited). This type of valve will overcome some of the problems in the conventional pilot but the response to this is relatively slow so that this system is probably only applicable for proportional control and not for a more demanding application such as closed looped servo control.
Examples of different electro hydraulic control systems are disclosed for example in U.S. Pat. Nos. 3,457,836 issued Jul. 29, 1969 to Henderson which employs a hydraulic motor in order to shift a spool; and 3,746,014 discloses a specific annular arrangement of a plurality of different pressure chambers utilized to control hydraulic flow.
The Gizeski U.S. Pat. No. 3,114,297 issued Dec. 17, 1962 discloses a system wherein a plurality of discrete interconnected pistons are used to move the main piston and thereby apply a selected pressure to a pressure control valve.
U.S. Pat. No. 4,768,544 issued Sep. 6, 1988 to Beam et al discloses a digital valve unit formed by a plurality of orifices sized in a binary relationship so that by selecting the proper combination or orifice sizes the desired flow can be obtained.
U.S. Pat. No. 4,391,092 issued Jul. 5, 1983 to Arnett describes a system similar to that described above with respect to U.S. Pat. No. 4,768,544 and utilizes the system to control the plurality of bleed air valves of the gas turbine engine.
U.S. Pat. No. 3,834,419 issued Sep. 10, 1974 to Bozoyan discloses a system of levers to digitally adjust the volume of hydraulic fluid in a bellows which is used to digitally adjust the position of a hydraulic control valve.
U.S. Pat. No. 4,503,888 issued Mar. 12, 1985 to Brovold uses digital rotary inputs from a stepper motor to digitally move a spool of a control valve.